U.S. Pat. No. 4,927,727, Rimai et al, issued May 22, 1990 and U.S. Pat. No. 4,968,578, Light et al, issued Nov. 6, 1990, describe a process for transferring one or more toner images to a receiving sheet in which the receiving sheet is heated prior to transfer. In some embodiments, the receiving sheet has a thermoplastic, heat softenable outer layer which is carefully heated, for example, by radiant heating prior to entering a nip, so that it is softened and is hot enough to sinter the toner contacting it at least where the toner particles contact each other. In this process, some of the particles of toner embed slightly in the thermoplastic layer and some of them do not. The ones that do not embed, sinter at the points of contact between the toner particles which is sufficient to transfer the toner without overall melting of the toner itself. Very high transfer efficiencies have been accomplished with this method.
U.S. patent application Ser. No. 07/405,258, TONER FIXING METHOD AND APPARATUS AND IMAGE BEARING RECEIVING SHEET, Rimai et al, and U.S. patent application Ser. No. 07/405,175, METHOD AND APPARATUS FOR TEXTURIZING TONER IMAGE BEARING RECEIVING SHEETS AND PRODUCT PRODUCED THEREBY, Aslam et al, are directed to fixing such a toner image by a combination of heat and pressure using a ferrotyping web and also to approaches for texturizing the image surface. In these applications, there is also disclosed the advantage of putting a curl preventing layer on the side of the receiving sheet opposite from the embedding layer, which curl preventing layer has a higher melting temperature than the layer in which the toner is embedded. The curl preventing layer is generally a thermoplastic, for example, a polyethylene, or polypropylene that has a relatively high melting point and therefore is less likely to offset when heated than would be a polyester or polystyrene or similar material used for the embedding layer.
The problem of blistering in a receiving sheet is a condition experienced in fusing apparatus. In general, moisture in paper will turn into steam as the temperature of the paper passes 100 degrees C. The steam expands and blisters an impervious outer layer on the paper that tends to stand in its way of escaping. Both the curl preventing layer and the thermoplastic embedding layer prevent the escape of steam and are therefore subject to blistering.
U.S. patent application Ser. No. 07/484,339 to Johnson et al, HEAT ASSISTED TONER TRANSFERRING METHOD AND APPARATUS, filed Feb. 26, 1990, discloses an improvement in the transfer process in which the heat for transfer is provided entirely from within the transfer roller. To prevent offset of the curl preventing layer and also to help reduce blistering, the transfer drum is entirely metallic which provides an extremely tight temperature control in heating the receiving sheets allowing temperatures associated with the thermoplastic layer to be in the 100 to 110 degrees C. range without overshoots of temperature that would cause melting of the curl preventing layer. Since the photoconductive layer is generally coated or wrapped on a metallic drum or roller, the transfer nip is a nip between two hard rollers or drums which is inordinately narrow for conventional transfer. However, contrary to expectations, the advantages gained with two metallic rollers exceeded the disadvantages of the narrow nip and superior results were obtained compared to transfer with a compliant outer surface on the transfer roller.